It has long been a customary practice for subscription television communication systems--a system in which the reception of a television program signal, generally recoverable by a group of television receivers, is limited to specifically authorized receivers--to scramble or encode their television transmission signals to prevent unauthorized reception by individuals that have not paid a subscription fee. Conventional hard-wired community antenna television systems (CATV), for example, are representative of a typical subscription television communication system, although other systems have been developed that employ radio frequency (RF) transmission to subscribers instead of hard-wiring. If the subscription television system provides more than one channel, it is also desirable to provide selected scrambling or securing of individual channels that are used to provide premium services.
While various methods have been suggested for scrambling a television transmission signal, one of the methods most commonly employed is to add a scrambling signal or interfering carrier signal to the television transmission signal between its aural and visual carriers. The scrambling signal is added to the television transmission signal to provide additional information which causes a television receiver to reconstitute the scrambled television transmission signal in an incoherent form. The scrambling signal is removed by passing the scrambled television transmission signal through a high-Q notch filter at the site of an authorized television receiver. In practice, an individual can obtain the high-Q notch filter by paying the required subscription fee.
There are inherent disadvantages to utilizing a notch filter or band elimination filter to remove the scrambling signal from the scrambled television transmission signal as described above. For example, a portion of the information content of the original television transmission signal, i.e. the signal prior to scrambling, is removed along with the scrambling signal causing signal degradation. Some of the degradation can be counterbalanced by utilizing a pre-emphasizing amplifier to symmetrically amplify a band around the scrambled signal as described in U.S. Pat. No. 4,074,311, but pre-emphasizing will not totally compensate for the degradation. In addition, it is difficult to provide a very narrow band filter at higher channel frequencies. Thus, degradation of the received television signal is more severe when securing higher frequency television channels.
It is an object of the present invention to provide a method and apparatus for securing a television transmission signal from unauthorized reception without requiring the use of a band-elimination filter at the site of an authorized receiver station. If the use of a band-elimination filter is avoided, the entire information content of the television signal can be provided to an authorized receiver, thereby avoiding the signal degradation problems associated with other conventional television security systems.